Quadrature amplitude modulation (QAM) is widely used for digital data transmission over radio links. With the new applications in inter-active technologies such as Digital TV and the Internet, QAM as a communications technology, will continue to be integrated into consumer electronic devices. Indeed, high definition television (HDTV) signals will be transmitted as compressed digitized data using QAM.
QAM transmits data as a sequence of two-dimensional complex symbols, i.e. with both in-phase and quadrature components. Each symbol adopts a specific pre-defined value based upon the data it represents. A set of all of the values available for transmission defines a character set which forms a constellation when graphically plotted on a two-dimensional basis.
The size and shape of the constellation depends upon the number of discrete values in the set and their spatial location in the constellation. The constellation proposed for use in broadcasting HDTV data contains, e.g., 16, 32 or 64 values.
To receive a QAM signal, a QAM demodulator within a receiver performs the functions of timing recovery, equalization and carrier recovery. Carrier recovery creates a reference carrier for determination of in-phase (i) and quadrature modulated (q) components, both in terms of frequency and phase, such that the received demodulated symbols do not rotate when the carrier is locked. It is the carrier signal that is quadrature modulated by the symbols and then transmitted to the receiver. Carrier recovery must be able to properly function in the presence of varying frequency offsets, drift and/or jitter that often occurs between a QAM transmitter and the receiver.
Through carrier recovery, a carrier frequency offset value is typically translated into a direct current (DC) value or digitized value that is used as a control input to a voltage or numerically controlled oscillator within a phase-locked loop. The output of this oscillator, being locked in frequency and phase to the reference carrier signal, is then used to extract, for example, baseband quadrature modulated information from the received signal.
In a QAM demodulator, a circuit is used to detect that the demodulator is locked, i.e., that the carrier and clock recovery loops are synchronized. This circuit is often called an Eye Quality Monitor ("EQM"). If it detects that the demodulator is unlocked, then the demodulator enters the acquisition mode and the carrier recovery circuit searches for the right carrier frequency, for example, by the use of a sweep circuit.
A carrier recovery circuit for a QAM demodulator is disclosed, for example, in U.S. Pat. No. 5,471,508 to Koslov. The carrier recovery circuit is operated in two modes: an acquisition mode to first attain an initial carrier lock, during which reduced constellation slicing is used; and a tracking (or lock) mode, during which full slicing is used to accurately track variations in frequency and phase that may occur to the carrier while the circuit remains locked.
Detecting the constellation size of a QAM signal is disclosed in U.S. Pat. No. 5,381,450. The probability density function of a received QAM signal is analyzed to determine the constellation size, e.g., 4, 16, 32, etc.
A problem can arise with the standard algorithms in that if the voltage controlled oscillator ("VCO") in the carrier recovery circuit oscillates with an offset frequency of exactly R.sub.s /4 (R.sub.s /4 is the Symbol Rate), or a multiple of R.sub.s /4, the demodulator would be erroneously declared locked. This is called a false-lock condition. It can occur when the symbol rate of the system is low, so that R.sub.s /4 is inside the locking range of the carrier recovery.
A conventional way to perform the EQM function is to examine the received constellation (FIG. 1). The received signal is sampled once per symbol at the optimum sampling instant, and if there are too many non-valid symbols during a certain period, the demodulator is declared unlocked. If the VCO in the carrier recovery oscillates with an offset of exactly R.sub.s /4, then the constellation rotates at R.sub.s /4. Accordingly, between each sample, it moves by exactly one quadrant and the received symbols are declared valid.